This invention relates to a direct-current arc furnace for steelmaking.
By the end of the 19th century it was known that the direct-current arc has substantial advantages as a local source of heat, as compared to the alternating-current arc. However, for a long time, there did not exist reliable and advantageous current rectifiers for the heavy-duty electric conditions of operation of arc furnaces. With the development of high-power diodes and thyristors during the 1960's, direct-current arc furnances became topical again.
The main problem of the direct-current furnances is the leading of the positive pole of the DC source to the molten steel. Prior to the 1960's, aproposed solution to this problem described bottom anodes on the basis of some hypothetic refractory, which is electrically non-conductive at normal temperature and achieves a sufficient electric conductivity at high temperature. See, e.g., British Pat. No. 118357 (1917). Such anodes have obviously not found any application, since such a refractory material does not exist.
Another proposed solution suggested movable water-cooled metal anodes which enter the furnace through holes in the walls, these holes being located higher than the door threshold. German Pat. DE No. 2340674. A drawback of such electrodes, however, is that metal and slag deposits in the wall holes block their motion and they become immovable, this resulting in a comparatively fast erosion.
Yet another proposed solution to the problem suggested cooled bottom metal anodes. Some designs describe a furnace bottom in which there are arranged a multitude of steel rods, the external bottom ends of which are cooled by air under forced circulation. See, 10th International Congress Electrothermy, Stockholm, 1984, Paper K-4. Recently, there has also been described a case where the anodes represent steel blocks built-in the furnace bottom, their bottom external ends being intensively water-cooled. See, 2nd Europ. Congress Electric Furnaces, Florence, 1986, Paper R-4.7. Such solutions are featured by the presence of a transient melting zone between the molten steel and the non-molten steel anodes. A drawback of such solutions is the arrangement of the anodes in the furnace bottom, this being a very vulnerable place. The deposition of the anodes in the area below the door or underneath the furnace spout does not provide advantages and only complicates the design. Such solutions are inevitably related to a sharply reduced {life} of the respective bottom components of the furnace.
There has been described a current supply by means of magnesite-carbon bricks, built-in the bottom portion of the furnace wall. See, 2nd Europ. Congress Electric Furnaces, Florence, 1986, Paper R-4.8. There has also been described the building-up of the furnace bottom with magnesite-carbon bricks. See, British Pat. No. 1504443. A drawback of such solutions is that, in similar conditions of operation, the magnesite-carbon bricks have a shorter life and do not ensure, in the beginning of the melts, the necessary electric conductance.
It is therefore a general object of this invention to develop a steelmaking arc furnace for operation with direct current, which utilizes the advantages of the DC arc and provides a good and reliable electric connection between the steel anodes and the molten steel.